First attempt at home-made decals...
Hi Max, it's called 'XXX main picture glue' if I remember correctly. I've only seen it sold in the USA but found some through a company called CML distribution that shipped/sold in the UK. It is specifically for gluing paper to the inside of lexan shells so probably not great for repairing existing stickers. One other note: somebody said that it is just PVA glue but I tried PVA first and the stickers fell off in a matter of hours! This XXX stuff has a lot of flex in it and has stuck perfect. Just be careful googling 'XXX..' :P Cheers!